The restoration of teeth frequently requires the placement of a core buildup filling material so as to produce adequate support prior to the construction of a crown. Teeth which have been the object of root canal treatment may additionally require the implantation of a post and/or pins prior to placement of the core buildup filling material so as to provide a suitable foundation prior to crown replacement or reconstruction.
Silicate cements have been used in the past for the repair of teeth. Silicate cement has the good properties of low thermal expansion, high abrasion resistance when not attacked by acids, and the ability to afford some caries protection by the liberation of fluoride ions.
Polycarboxylate cements are noted for their hydrophilic properties, good adhesion to tooth structure and apparent blandness. Polycarboxylate cements are based on zinc oxide, or zinc oxide and magnesium oxide or tin oxide and an aqueous solution of polyacrylic acid or an acrylic acid copolymer with other unsaturated carboxylic acids. Other additives may include silica, or alumina and bismuth salts, or stannous fluoride, or tannic acid. In the setting reaction, zinc polyacrylate is formed together with other metal polycarboxylates. These materials, however, have insufficient strength to be used as a core material.
The two most widely used and clinically effective core buildup filling materials are silver amalgam and composite resins. Silver amalgam exhibits high strength, usually in the range of from 300 to 400 MPa (megapascals). Both silver amalgam and composite resins possess coefficients of thermal expansion which are two to three times that of tooth material. This is a significant disadvantage and may result in increased microleakage and may lead to recurrent caries and/or solubility of the post luting cement. Other disadvantages of the silver amalgam and the composite resins include possible corrosion of the amalgam at the post and core material interface and lower compressive strength of the composite resin. Composite resin and amalgam also frequently require the placement of pins in conjunction with a post in order to obtain adequate retention of the core buildup material. Furthermore, both materials lack the ability to provide desirable chemical bonding to dentin, which may further increase the propensity for microleakage.
Glass ionomer cement filling materials have been previously developed which have addressed some of the above disadvantages of amalgam and composite resin. Glass ionomer cement has strength characteristics similar to those cited above for silicate cement but is more resistant to acid attack. It is also bland, like the polycarboxylate cements, but with the added advantage of translucency.
Glass ionomer cements utilize the hardening reaction between ion-leachable glasses and aqueous solutions of a polymeric binder such as homo- and copolymers of acrylic acid and/or itaconic acid. When the glass and the binder are mixed, H.sup.+ ions from the acid of the liquid penetrate the surface layers of the glass particles. Cations including Al.sup.+3 and Ca.sup.+2 are displaced and the aluminosilicate network of the glass surface is degraded to a hydrated siliceous gel. Cations, either simple or as fluoride complexes, migrate into the aqueous phase of the cement paste where metallic salt bridges are formed between the long chains of charged polycarboxylate ions, crosslinking them and causing the aqueous phase to gel and the cement to set. Calcium ions are more rapidly bound to the anion of the polymeric binder than are aluminium ions and it is the calcium ions which are responsible for the initial set. Subsequent formation of the aluminum salt bridges accounts for the hardening of the cement. Thus a dual setting reaction exists for glass ionomer cements consisting of a calcium ion-exchange (initial setting) and an aluminium ion-exchange (final hardening setting). Adhesion between the glass ionomer adhesive and a tooth substrate results from the dipole and ionic interactions because glass ionomer cements and the substrates having a polar nature.
The glass ionomer cements generally consist of, for example, a polycarboxylic acid material and glass, such as finely ground aluminosilicate glass. Such glass ionomer cements will adhere to the dentin of the tooth without the need for a bonding agent or primer or coupling agent. In addition, these cements are able to provide a source of desirable flouride ion leachable from the glass. Flouride ion has been shown to be an effective agent in the prevention of caries. Also, glass ionomer cements are generally biocompatible with the dental tooth pulp. However, some glass ionomer cements are sensitive to water as evidenced by a reduction in adhesion and/or compressive strength over time upon exposure to moisture. Furthermore, the tensile strengths in general of unreinforced glass ionomer cements are not sufficient. These disadvantages make the unreinforced glass ionomer cements unsuitable as core buildup materials.
It is known to improve the tensile and/or compressive strengths of the glass ionomer cements used as a core buildup material by the addition of certain metallic fillers, such as silver, or silver alloy powders, such as dental amalgam. This results in certain improved properties which make the reinforced cement useful as a core buildup material for the restoration of teeth prior to crown replacement. These properties include: (1) a coefficient of thermal expansion similar to that of tooth structure; (2) increased strength when compared to conventional unreinforced glass ionomer cements; and (3) the release of fluoride ions to adjacent teeth. Commercially available silver reinforced glass ionomer cements have compressive strengths of approximately 168 to 175 MPa. The addition of silver or silver alloys to the glass ionomer cement produces increased strength when compared to unreinforced glass ionomer cements.
It has been shown, however, that simple mixtures of metal powders and aluminosilicate glass ionomer powders often fail to provide any metal/polyacrylate bond, which subsequently can lead to increased wear of the filling material.
McLean and Gasser have recommended the use of a sintered cermet glass/metal composition to increase the bond strength between the glass and various metal fillers and to decrease abrasion resistance. In glass-cermet cements, the glass and metal powders are sintered or fused to high density, ground, and the powdered mixture then combined with acids to form the final cements. Several of the precious metals well-known in dentistry may be used in the preparation of glass-cermet cements, but gold and silver are the most suitable. Cermet cements differ from simple mixtures of metal and glass powders since the metal powder is firmly bonded to the glass by high temperature sintering.
McLean et al., Quintessence International, volume 16, page 333, (1985), teach the use in cermet cements of various metals including alloys of silver and tin, pure silver, gold, titanium, and palladium. Gold and silver were disclosed as metals which form suitable cermets with the aluminosilicate glass. However, the compressive and/or tensile strength of these metal-reinforced glass ionomer cements was still inadequate for high stress tooth areas.
Brown and Combe (J. Dent. Res. March-April 1973, vol. 52 No. 2, page 388) have reported the use of stainless steel in polycarboxylate cement. The stainless steel was chosen therein for its supposed ability to form an adhesive bond with this type of cement. This study did not use glass ionomer cements which employ significantly different chemistry than is employed in polycarboxylate cements. Furthermore, the Brown and Combe study relied, for the setting of the cement, on the presence of zinc ions from the zinc oxide used in polycarboxylate cements.
Brown and Combe (J. Dent. Res., vol. 50 page 690, 1971), reported the investigation of stainless steel as a potential reinforcing agent for zinc polycarboxylate cements. The study did not use glass ionomer cements, but instead used zinc oxide and polycarboxylate. The study concluded "that polycarboxylates are too brittle to be improved substantially by metallic fillers in accordance with the theory of reinforcement".
Therefore, a need exists for an improved metal-reinforced glass ionomer dental material which produces a more economical, very strong, fracture resistant dental core filling material. The metal reinforced cement should be applicable in all situations in which the commercially available cements are utilized, including among others: (1) core buildup filling material prior to crown preparation; (2) core filling material in combination with a prefabricated post to restore root canal treated teeth; (3) temporary or permanent filling material of primary or permanent teeth or dental implants, and (4) a luting agent for the cementation of permanent dental restoratons.